The dehydrogenation reaction of a carrier medium is endothermic, i.e. heat has to be supplied. This can be effected by means of a shell-and-tube reactor in order to make a ratio of surface area to volume of the reactor required for introduction of heat possible. The ratio of surface area to volume is greater, i.e. better, the smaller the respective tube diameter. However, a small tube diameter means a reduced tube cross section, so that the liberation of hydrogen gas is hindered.